Effect of canopy biomass and wave exposure on growth in Laminaria hyperborea (Laminariaceae : Phaeophyta)

Annual growth per plant and allocation of growth to the stipe were measured in 2- to 4-year-old plants of Laminaria hyperborea from five stations with different degrees of wave exposure and different amour:ts of canopy biomas s. Low growth in plants with initial stipe lengths of less than about 40 cm suggested that the presence of canopy-forming plants suppresses growth of understorey plants, and this was supported by the high lamina growth rate o f understorey plants after removal of the canopy-forming plants. Canopy bio mass and wave exposure were found to exert a differential effect on the gro wth of the age groups examined, Average annual growth per plant in 2- to 3- year-old plants decreased with increasing canopy biomass; growth of 4 year- old plants was not significantly influenced by canopy biomass, but increase d with increasing wave exposure of the sites, suggesting an influence of so me factor connected with wave exposure. The allocation of annual growth to stipe and lamina was also found to be influenced by canopy biomass and wave exposure. In 2- to 3-year-old plants the amount of annual growth allocated to the stipe increased with increasing canopy biomass while a positive lin ear regression between allocation of growth to the stipe and wave exposure was found in 4-year-old plants. Individual measurements carried out at a wa ve-exposed locality showed that the maximum allocation of growth to the sti pe in the intermediate-sized plants occurred simultaneously with a period o f rapid stipe elongation. Thus, rapid stipe growth in L. hyperborea can be associated with high allocation of annual growth to the stipe when the plan ts are about to grow into the canopy-forming layer. We suggest that this is due to the combined effects of suboptimal light levels within the kelp for est and high wave exposure.